Ethylene is the leading petrochemical in terms of production volume, sales value and number of derivatives. Total worldwide ethylene production in 1995 was estimated at 76 million tonne per year, expectations are for a growth rate of about 3% per year, and meeting this growth will require significant capital investment in new production facilities. Sales prices average on the order $440/tonne, translating to a worldwide cash volume for the ethylene business of over $30 billion per year. End and intermediate uses of ethylene include production of plastics, resins and fibers, and a host of other products.
Before ethylene can be sold or used, it is necessary to employ a process which recovers the ethylene component in a desireable, ethylene rich product stream by separating it from a myriad of other components, including methane, ethane, hydrogen and carbon monoxide, among others, all of which components are found together in a single stream obtained from another, different olefin generation/preparation process. Currently, a desireable, ethylene rich product stream is generally defined by those skilled in the art as one having greater than about 95 wt. % ethylene, containing substantially inert components such as methane and ethane in proportions less than about 2000 molar ppm each and potentially reactive components such as hydrogen, carbon monoxide, carbon dioxide, propylene and others in proportions less than about 20 molar ppm each. This definition arises from the nature of the derivative processes that use the ethylene rich product stream, each suffering varying degrees of adverse process performance and economic impact associated with the levels of the various non-ethylene components in the stream. Such material shall hereinafter be referred to as a primary ethylene rich product stream.
The recovery and separation process which produce s a primary ethylene rich product stream from components received from an olefin generation/preparation process represents the majority of total capital investment and energy usage required for ethylene manufacture. This reflects the difficulty associated with techniques required to manage the low normal boiling points and low relative volatilities of ethylene and the other components received from an olefin generation/preparation process. Furthermore, as recognized by those skilled in the art, capital recovery and energy usage are generally the two largest cost elements, respectively, in the total cost of ethylene manufacture. Thus, the process by which one effects recovery and separation to produce a primary ethylene rich product stream is a substantial factor in the economic feasibility of ethylene manufacture.
Methods for the recovery and separation of ethylene found in multi-component streams have been under consideration since the 1940's, when the first practical, large scale olefin generation technique of hydrocarbon pyrolysis, also called steam cracking, was developed and applied commercially. This olefin generation technique, now a substantially mature art, currently dominates the industry, utilizing a number of different hydrocarbon feedstocks. Also, alternative processes of potential commercial significance for the generation of olefins are emerging, such as the methanol to olefins process, taught in Kaiser, U.S. Pat. No. 4,499,327, and now being offered for commercial license by UOP. As shown in Table 1, olefin generation techniques of commercial importance, in general, create differing quantities of various byproduct components in a mixture with the desired ethylene component.
TABLE 1 ______________________________________ Typical Component Distribution From Various Olefin Generation Techniques Steam Cracking Methanol Process (1) To Olefins Feedstock Light Naphtha Atmospheric Gas (2) (excluding Ethane (boiling range Oil (boiling range Methanol water) (C.sub.2 H.sub.6) 95-300.degree. F.) 365-635.degree. F.) (CH.sub.3 OH) ______________________________________ Yields, wt. % (excluding water) H.sub.2 3.9 1.00 0.6 0.03 CO trac trace trac 0.49 CO.sub.2 trac trace trac 2.46 CH.sub.4 3.8 18.00 11.2 1.45 C.sub.2 H.sub.2 0.4 0.95 0.4 0.00 C.sub.2 H.sub.4 53.0 34.30 26.5 53.73 C.sub.2 H.sub.6 35.0 3.80 3.4 1.67 C.sub.3 H.sub.4 0.0 1.02 0.8 0.00 C.sub.3 H.sub.6 0.8 14.10 13.4 26.37 C.sub.3 H.sub.8 0.1 0.35 0.2 1.53 C.sub.4 H.sub.6 1.1 4.45 5.0 0.00 C.sub.4 H.sub.8 0.1 3.70 3.7 6.64 C.sub.4 H.sub.10 0.2 0.10 0.1 1.21 C.sub.5 0.2 2.75 2.7 3.37 C.sub.6 -C.sub.8 0.3 1.20 1.2 0.88 benzene 0.3 6.90 6.9 0.00 toluene 0.0 3.20 3.2 0.00 xylene + 0.0 1.30 1.3 0.00 ethylbenzene styrene 0.0 0.79 0.7 0.00 C.sub.9 -400.degree. F. 0.0 2.96 2.9 0.00 fuel oil 0.0 15.45 15.4 0.00 carbon trac trace trac 0.17 Total 100.0 100.0 100.0 100.00 ______________________________________ Note: (1) per HoweGrant, M. Ed., Encyclopedia of Chemical Technology, fourth edition, Volume 9, page 880 (1994) (2) per Nirula, S. C., Ethylene from Methane, Stanford Research Institute International Process Economics Program Report No. 208, page 42 (1994)
This mixture is generally unsuitable for further commercial use, hence the need for a distinct recovery and separation process. It should be stated that this is the case for many other olefin generation techniques not shown in Table 1, and for blends of mixtures from those techniques, one commercially important example being off-gas mixtures generated in various refinery processes blended with mixtures created in steam cracking. Further, such mixture may be a blend of those created by the olefin generation process and recycle streams from other parts of an ethylene manufacturing or ethylene derivative manufacturing facility, and may contain components other than those listed in Table 1.
The prevailing conventional wisdom relating to ethylene manufacture directs one, in addition to creating ethylene and byproduct components in a mixed stream via an olefin generation technique or blend of techniques, to further prepare that stream for introduction to the subsequent recovery and separation process. This may include, in various embodiments and sequences, the actions of cooling the stream from conditions at which the olefin generation reaction is effected to near ambient conditions, compressing the normally gaseous mixed stream to pressures usually between 200 and 600 psia, removing almost all of the water, carbon dioxide and sulfur compounds used or produced in the olefin generation step, and removing various normally liquid components at various pressures from the mixed stream. Hence the combination of steps described above, namely those of olefin generation and olefin preparation, embody what is referred to herein as the olefin generation/preparation process. Those skilled in the art recognize the result of employing an olefin generation/preparation process is production of a stream known as "charge gas," so named because it is both the mixed component gaseous charge to and from large and expensive compressors within the process, and the mixed component gaseous charge to the subsequent recovery and separation process. This latter stream will hereinafter be referred to as pressurized mixed olefin bearing charge gas.
Though methods for the recovery and separation of ethylene from a pressurized mixed olefin bearing charge gas are known, the low normal boiling points of ethylene and other components require the use of very low temperature vapor-liquid flash and fractional distillation techniques, in order to have a high recovery of the ethylene molecules present in a primary ethylene rich product stream, and thereby render ethylene manufacture sufficiently efficient for economic viability. Of particular expense in these processes are equipment items which serve to separate ethylene from lower boiling components such as hydrogen, carbon monoxide and methane. In current state of the art ethylene recovery and separation processes which dominate the industry, temperatures on the order of -60 to -215.degree. F. are typically employed in certain equipment items, requiring special metallurgies and refrigeration systems to effect, which represent a substantial portion of the total capital cost and energy consumption of ethylene manufacture. These are generally known to those skilled in the art as the chill train and the demethanizer tower, and a substantially dedicated refrigeration system needed to operate those equipment items at the requisite low temperatures, usually using ethylene as the refrigerant but sometime using methane and mixtures of light hydrocarbons, among other refrigerants. Also found in the recovery and separation process to produce an ethylene rich product stream are deethanizer and C.sub.2 splitter fractional distillation towers, and reactors to eradicate the presence of acetylenes and dienes, along with heat exchangers, pumps and other supporting equipment items.
A less frequently practiced alternative to recovery and separation of ethylene from methane and lower boiling components using temperatures in the range of -60 to -215.degree. F. is comprised of employing a combined absorption and fractional distillation technique in the demethanizer tower, known to those familiar with the art as an absorber demethanizer. In this technique, a large volume of substantially ethylene free material of higher boiling point than ethylene, called lean oil, is introduced to the absorber demethanizer above the feed tray(s), usually the condenser drum or top tray, in the liquid state at about -20 to -50.degree. F. The dominant quantity of lean oil provides the bulk of the molecules in the vapor phase of the resulting overall vapor liquid equilibrium among all the components, and thus serves to force the bulk of the ethylene into the liquid phase, effectively absorbing it. The methane and lower boiling components, being more volatile, still tend to remain in the vapor phase, and thus over the course of numerous trays in the absorber demethanizer tower, separation of ethylene is effected as the lean oil with absorbed ethylene moves down the tower to the bottoms, and methane and lower boiling components move up the tower to the overhead. The absorber demethanizer has the potential advantage of eliminating the chill train, the substantially dedicated refrigeration system and some of the special metallurgies needed to operate at temperatures below -55.degree. F. However, with most types of pressurized mixed olefin bearing charge gas these advantages are offset by associated increases in high temperature refrigeration loads, energy consumption and cooling equipment items to manage the heat of absorption of ethylene in the lean oil within the absorber demethanizer. Further, additional size and energy consumption is required in subsequent distillation towers in the overall recovery and separation process to separate the large volume of lean oil from the desired ethylene and other byproduct components. For a representative example, see U.S. Pat. No. 5,019,143 granted to Mehra, et. al.
An advancing technology in the field of ethylene recovery and separation processes is that of non-distillative and non-cryogenic techniques, especially those that serve to separate olefins from non-olefins. One practical example is that of chemical absorption and desorption, such as the use of aqueous silver nitrate solutions in the British Petroleum "Selective Olefin Recovery" technology, described by Barchas in his conference presentation entitled Olefin Recovery Via Chemical Absorption and currently being offered for license by Stone and Webster, Inc., and which operates at ranges between about 600 psia/70.degree. F. and 2 psia/400 for absorption and desorption, respectively. Another is the use of membrane separators, such as described in U.S. Pat. No. 5,082,481 to Barchas, et. al. to remove approximately 20% of the hydrogen from a pressurized mixed olefin bearing charge gas prior to any refrigeration of the charge gas. These techniques have the advantage of a low capital cost per unit of ethylene processed, yet at present, they are incapable in and of themselves to transform a sufficient quantity of ethylene in the pressurized mixed olefin bearing charge gas into an ethylene rich product stream of sufficient purity for economic ethylene manufacture. They can be synergistically combined with flash and distillation equipment to produce some advantageous results, such as described in U.S. Pat. No. 5,452,581 to Dinh, et. al., wherein membranes are used to remove hydrogen in the chill train, thus saving energy by moving refrigeration load from the substantially dedicated low temperature refrigeration system to a high temperature refrigeration system. However, they cannot entirely eliminate the chill train, or low temperature refrigeration system, or fractional or absorptive distillation. Rather, their highest value seems to be in applications on relatively low volume secondary ethylene containing streams, such as process purges that are high in ethylene content in the case of chemical absorption. Hereinafter, such equipment and techniques shall be called bulk separation techniques.
Regardless of the specific embodiment of a recovery and separation process employed in ethylene manufacture, the vast majority of such processes are, in the conventional wisdom, directed to producing a single, primary ethylene rich product stream suitable for all possible end uses in the subsequent manufacture of ethylene derivatives. The dominance of processes producing a single, primary ethylene rich product stream is historically driven by polyethylene manufacture, which comprises the majority of the overall use of ethylene for derivative manufacture, and which has the most stringent specifications of all derivatives, usually requiring an ethylene rich product stream of high purity.
However, the prevailing conventional wisdom with respect to the composition requirements of the resultant primary ethylene rich product stream for derivative manufacture is just now being tested in the industry. In the Purvis conference presentation entitled Cracker/Derivative Unit Integration, there is discussed the recognition that many derivative processes do not require the historically high levels of ethylene purity required by that of polyethylene to function adequately. One such process is for ethylbenzene manufacture, as disclosed in U.S. Pat. No. 5,476,978 to Smith, et. al., which states the ethylene rich product stream used as feed may contain ethylene in concentrations as low as 5 wt. %, and such process as which is offered for license by CDTech, Inc. of Texas claims the ethylene rich product stream may contain appreciable levels of hydrogen and carbon monoxide. Another derivative process is that for aldehyde, alcohol, or ester manufacture, as described in European Patent Application serial number PCT/EP96/00361 by Kiss, et. al., wherein suitable feeds include an ethylene rich product stream that need contain only between 30 and 75 wt. % ethylene, and which may contain appreciable quantities of hydrogen and carbon monoxide. Further, the question of the purity of a primary ethylene rich product stream for use in polyethylene manufacture itself is being broached, with concentrations of ethylene as low as 85% being discussed. The reason for entertaining the production and application of a primary ethylene rich product stream lower in ethylene content than historically prevalent is to allow reductions the capital and energy requirements associated with producing the primary ethylene rich product stream, and ultimately provide reductions to the cost of the ethylene bearing derivative product.
Yet, in the face of the changing requirements for primary ethylene rich product streams for the manufacture of various derivatives, almost all current teachings in patents and open literature continue to direct one to make a single, primary ethylene rich product stream from the recovery and separation process. Those few items that refer to a different, secondary ethylene rich product stream teach of the stream emanating from the deethanizer fractional distillation tower to save additional load on the C.sub.2 splitter tower. As far as applicant is aware, it is heretofore unknown to use a demethanizer tower to supply as an overhead a secondary ethylene rich product stream for potential use in certain derivative processes, or for feed to advantageous applications of the advancing art in low cost non-distillative and non-cryogenic separation techniques, while continuing to supply a primary ethylene rich product stream with more stringent composition requirements for other derivatives, and simultaneously reducing capital costs and energy requirements for the overall separation and recovery process, potentially eliminating the chill train and dedicated low temperature refrigeration system, or alternatively potentially eliminating the circulation and management of lean oil to the demethanizer tower.